Type II topoisomerases are ubiquitous enzymes that are required for proper chromosome structure and segregation and play important roles in DNA replication, transcription, and recombination. Type II topoisomerases relax DNA and remove knots and tangles from the genetic material by passing an intact double helix (transport segment) through a transient double-stranded break that they generate in a separate DNA segment (gate segment). Humans encode two closely related isoforms of the type II enzyme, topoisomerase II and topoisomerase II. Topoisomerase II is essential for the survival of proliferating cells and topoisomerase II plays critical roles during development. However, because these enzymes generate requisite double-stranded DNA breaks during their crucial catalytic functions, they assume a dual persona. Although essential to cell survival, they also pose an intrinsic threat to genomic integrity every time they act. Beyond their critical physiological functions, topoisomerase II and II are the primary targets for some of the most active and widely prescribed drugs currently used for the treatment of human cancers. These agents kill cells by increasing levels of covalent topoisomerase II-cleaved DNA complexes that are normal, but fleeting, intermediates in the catalytic DNA strand passage reaction. Most clinically relevant drugs do so by inhibiting the ability of the type II enzymes to ligate cleaved DNAs. When the resulting enzyme-associated DNA breaks are present in sufficient concentrations, they can trigger cell death pathways. Anticancer drugs that target type II enzymes are referred to as topoisomerase II poisons because they convert these indispensable enzymes to potent physiological toxins that generate DNA damage in treated cells. Although topoisomerase II and II are important targets for cancer chemotherapy, evidence suggests that they also have the potential to trigger specific leukemias. A small percentage of cancer (and other) patients treated with topoisomerase II-targeted drugs eventually develop acute myeloid leukemias (AMLs) involving the MLL gene at chromosome band 11q23 or acute promyelocytic leukemias involving 15:17 translocations. The 11q23 chromosomal translocations also are seen in infant AMLs, and the risk of these leukemias rises ~3-fold when there is high maternal exposure during pregnancy to environmental and dietary topoisomerase II poisons. Despite the importance of the type II enzymes to cell growth and cancer, interactions between human topo- isomerase II and DNA, anticancer drugs, and other topoisomerase II poisons have not been well characterized. Thus, the aims of this proposal are to further define the catalytic mechanism of topoisomerase II, to further delineate the mechanism by which topoisomerase poisons increase levels of enzyme-mediated DNA breaks, and to determine the cellular consequences of topoisomerase II poisons. The primary research model for this study will be human topoisomerase II and II. Bacillus anthracis gyrase and topoisomerase IV will be used for some experiments to provide comparisons between the prokaryotic and eukaryotic type II enzymes.